1. Field of the Invention
The present invention pertains to the medical imaging field and more particularly to a scanning-liquid ionization chamber (SLIC) IMAGER/DOSIMETER for megavoltage photons.
2. Background of the Invention
The process by which a patient is exposed to a small amount of radiation after being positioned on a treatment couch but before the main treatment for purposes of assuring the correct positioning of the patient is known as localization imaging. During the course of the radiation treatment, it is desirable to be able to verify that the patient has not moved and is still in the desired position--this is known as verification imaging. The present invention can be used for both types of imaging.
At the present time, virtually all localization and verification imaging is performed using film. This results in several minutes of time being expended to produce a single image due to the inherently time-consuming development process. Further, film offers no real-time imaging capability so that only composite verification images integrated over the treatment can be produced. Imagers based on storage phosphor technology have become commercially available; however, the storage-phosphor imagers do not function in real time.
For a patient undergoing radiation therapy, time delays between irradiation and image formation can be wrought with a number of undesirable consequences. In the case of localization imaging, time delays result in patient discomfort. More importantly, time delays can result in set-up error caused by patient movement. The undesired exposure of healthy tissue to radiation is one consequence of set-up error. Another consequence is the difficulty of ascertaining the exact quantity of radiation which a target area has received.
Several prototype real-time imagers are being developed around the world, but most have no practical applications to clinical use. The most promising realtime clinical image detector in the literature to date is that developed by H. Meertens at the Netherlands Cancer Institute in Amsterdam and disclosed in European Patent Application 0196138. Related articles concerning Meertens' imaging device are M. Von Herk and H. Meertens, Radiotherapy and Oncology, 11 1988, pp 369-378, and H. Meertens et al, Phys. Med. Biol., 1985, Vol. 30, No. 41 pp 313-321.
The Meertens' device operates on the principle of a scanning liquid ionization chamber. The chamber is filled with a liquid dielectric, e.g. trimethylpentane pure to approximately 50 ppm, which acts as the ionization medium.
A problem with the Meertens' device is that it detects only positive and negative ions formed by the ionization radiation and not electrons. The reason for this inability to detect electrons lies in the fact that the ionization medium used by Meertens has a contamination level which results in the electrons being trapped by impurities in nanoseconds.
For electron detection an ionization medium having only a few molecules of impurities per billion molecules of ionization medium is desired. In this patent, impurities are understood as being electronegative impurities. However, the circuit-board design of the Meertens' device prevents such a level of purity from ever being attained. Contaminants inherent to the Meertens' circuit board pollute any liquid ionization medium to an unacceptable degree immediately upon the liquid's introduction to the device. This is to say that a very small portion of the materials constituting the Meertens' circuit board are dissolved in the liquid ionization medium. However, even this small portion of contamination makes electron detection impossible. Furthermore, the Meertens' ionization medium is subject to contamination by air leaking through the detector walls which are too porous for maintaining the necessary degree of purity.
Advances in detector technology at CERN in Geneva, Switzerland have resulted in radiation detectors which use parts per billion clean 2,2,4,4-tetramethylpentane (TMP) as an ionization medium. TMP is now realized to be a superior ionization medium, see Nuclear Instruments and Methods in Physics Research A265 pp 303-318. The CERN detectors have been designed for experiments in high-energy physics and are not suited for or adaptable to the field of medical scanning, e.g. the CERN detectors are exposed to ultra-high energy particles of many billions of electron volts for purposes of generating showers of high energy particles. The CERN detectors exhibit relatively thick electrodes which do not necessitate great precision in their spatial relationships. However, the box design of the detectors used at CERN have proven effective for maintaining TMP at what researchers believe is a few parts-per-billion clean level after months of use.
Thus, a need exists for a scanning liquid ionization chamber which can house and maintain a liquid ionization medium which has less than 100 molecules of impurities per billion molecules of ionization medium, resulting in reduced signal extraction time and improved signal-to-noise ratio. (Although an ionization medium having fewer than 100 molecules of impurities per billion molecules of ionization medium is stated as being desired, what is meant by this is that an ionization medium is desired which has a purity level which allows electron lifetimes to exceed 100 microseconds. Without question a high correlation exists between the purity level of an ionization medium and the resultant electron lifetime. Although it is at present difficult to quantify the purity level of a liquid ionization medium to a part-per-billion accuracy, the physics inherent to the present invention indicate that fewer than 100 molecules of impurities can be present in one billion molecules of ionization medium if electron lifetimes are to exceed 100 microseconds.)